Thermally-responsive electrical switches of the sliding contact-type are generally known. One type of such switch is shown in the Merrill U.S. Pat. No. 3,180,958, issued Apr. 27, 1965. In the switch of the Merrill patent, a pair of axial leads are used to connect the device into an electrical circuit. One of the axial leads is electrically connected to an electrically conductive, cylindrically-shaped housing and the other lead is positioned into the interior of the housing through an electrical insulator which electrically isolates it from the housing. A coil spring is positioned around a portion of the electrical insulator to make contact with a slideable contact member that engages the inner surface of the electrically conductive housing. A solid, thermally-sensitive pellet is positioned at the end of the housing which is connected to the uninsulated axial lead. A separating washer is positioned between the thermally-sensitive pellet and a second spring which is inserted between the washer and the sliding contact. The second spring acts to force the slideable contact against the head of the insulated lead to complete the electrical circuit from the outer electrically conductive housing through the insulated lead as long as this pellet remains solid. When the pellet melts, the coil spring forces the slideable contact away from the insulated lead thereby breaking electrical contact between the leads.
In the switch of the Merrill patent the washer against the pellet has a diameter which is less than the diameter of the pellet. This is a requirement of the Merrill switch since when the pellet melts the liquid mass of the pellet must flow around the washer and over the second bias spring in order to allow for the required movement of the slideable contact. This combination of an undersized washer and the second spring makes the assembly of the device somewhat more complicated than desirable, and the device of the Merrill patent is somewhat position-dependent since the flow of the melted mass of the pellet material around the separating washer will be in part dependent upon the force of gravity. The flow of the melted mass of the pellet of the Merrill patent, therefore, will vary somewhat according to the physical orientation of the device. The improved device of the present invention is easily assembled and almost completely position-independent since operation of the switch does not rely upon the pressure supplied by a separate secondary spring to force the melted mass of the pellet around itself and an undersized washer, as does the switch of the Merrill patent. Moreover, the device of the present invention is anticipated to be cheaper to make and of a higher reliability.
Another reason for the anticipated cost savings and increased reliability is that in the device of the present invention the spring forces of the coil spring and the spring fingers that are integrally formed from the sliding contact member are independent of one another. In the Merrill patent there is a space between the pellet and the head of the insulated lead, and the design therefore requires that two springs be used, but one spring must have a stronger force constant than the other since the stronger spring must overcome the secured spring to maintain electrical contact. In the device of the present invention, by contrast, the pellet itself, either alone, or in conjunction, with a separating washer, directly acts on the spring fingers to hold them in contact with the head of the insulated lead; and thus the spring force of the spring fingers does not have to overcome the spring force of the coil spring.